BACKGROUND: Phenobarbital is one of the most commonly used drugs to treat epilepsy and other seizure disorders in dogs and cats. Hepatotoxicity following phenobarbital administration is dose-dependent. OBJECTIVES: The present study aimed to evaluate the protective action of silymarin on phenobarbital-in- duced hepatotoxicity in cats. METHODS: For this purpose, twenty-four healthy adult cats were randomly allotted to four equal groups. Cats in group A were given phenobarbital with dosage 16 mg/kg orally for 28 days; group B received silymarin (30 mg/kg/day for twenty-eight days) orally concurrent with phenobarbital; groups C and D were treated like group B, but silymarin was administered 3 and 48 h after administration of phenobarbital, respectively, and continued up to twenty-eight days. The serum concentrations of alanine aminotransferase, aspartate amino- transferase, alkaline phosphatase, lactate dehydrogenase, total and direct bilirubin, blood urea nitrogen and creatinine were measured before administration of phenobarbital and after 24 h, 72 h and 28 days. RESULTS:Phenobarbital elevated significantly serum concentrations of liver enzymes (in all cases), and total and direct bilirubin in two cats of group A, after 24 h (P <0.001). In groups, B and C, levels of serum enzyme activities and total and direct bilirubin remained within the normal range up to 28 days (P>0.05), while in group D, levels of serum enzyme activities (in 4 cases) were higher than the normal values (P<0.001). CONCLUSIONS: The results showed that silymarin can protect liver tissue against oxidative stress in cats with phenobarbital intoxication especially in the first 3 h post-exposure.